I can't help on Q#1 but for Q#2, you have to turn on and choose one two-layer model in the 'turbulence models' tab, and turn on the 'Two-layer' option to 'Active' for the walls you want to use the model on. You can leave the other walls as normal (Two Layer = inactive) to use the 'regular' wall-function models on them to save time on calculations (eg. on areas of less interest).